1. Field of the Invention
The present invention relates to a noise figure-measuring device and a noise figure-measuring method.
2. Description of the Related Art
In recent years, an optical amplifier has been promoted to improve the quality of transmission information in an optical communications system. The signal to noise ratio of transmission information according to the optical communications system using an optical amplifier is strongly influenced by noise characteristics of the optical amplifier. Therefore, when constructing an optical communications system using an optical amplifier, it is necessary to accurately grasp a noise figure NF, which is a basic characteristic of an optical amplifier. For that purpose, the level of amplified spontaneous emission (ASE) light contained output light from the optical amplifier must be accurately measured. The optical amplifier outputs input light after amplification thereof. A noise component contained in the output light from the optical amplifier is ASE light. In the conventional art, during measurements of an ASE light level, a fitting of a higher-degree function such as a normal distribution curve, a Lorenz curve or the like is performed with respect to a wavelength region excluding an output signal light component of an optical amplifier and the level for a signal light wavelength of an obtained approximation curve is specified as an ASE light level.
Herein, referring to FIG. 7, a method for measuring an ASE light level in the conventional art will be described.
FIG. 7 is a flowchart for explaining an ASE light level measurement in the conventional art.
Input signal light having a signal wavelength λS is amplified by an optical amplifier, the amplified input signal light is outputted to an appointed optical spectrum analyzer. The appointed optical spectrum analyzer generates output light spectrum P2(λ) data from the optical amplifier. Of the output light spectrum P2(λ) data thus obtained, output light spectrum P2(λ) data within a wavelength range of λS±ΔλMA, that is, a wavelength mask range of ±ΔλMA (which has been set by a user in advance) around the center of the signal wavelength λS is masked (removed) (Step S71).
After the above-described mask process, a curve-fit process using an appointed fitting function is performed for the output light spectrum P2(λ) data that has not been masked. A spectrum within the above-described masked wavelength range of λS±ΔλMA is interpolated, and then an ASE light spectrum P3(λ) is specified (Step S72).
The above-described fitting function is a high-degree function such as a normal distribution curve, a Lorenz curve or the like and has been selected by the user in advance.
An ASE light level P ASE on the signal wavelength λS is determined based on the ASE light spectrum P3(λS) (Step S73).
However, in the aforementioned conventional method, there have been the following problems. A measurement of the noise figure NF of the optical amplifier is performed based on the ASE light level. Accordingly, an accurate measurement of the ASE light level is strongly requested. In addition, a light signal to be inputted in the optical amplifier is a laser beam owing to a light source such as a DFB-LD, etc. and in terms of this light source spectrum, in addition to a signal light wavelength component, a source spontaneous emission light (SSE) component is contained in a noise component thereof. Therefore, in noise components output from the optical amplifier in addition to ASE light, an amplified SSE light component is also contained.
As shown in FIG. 8A, the source spontaneous emission light component SSE is contained in input light spectrum P1(λ) data. Therefore, an amplified source spontaneous emission light component SSE is also contained in output light spectrum P2(λ) data. Namely, in the output light spectrum P2(λ) data, , the amplified SSE light is contained in addition to the amplified signal light and ASE light.
Therefore, when measuring the ASE light level based on the output light spectrum of the optical amplifier, a composite light level between the ASE light and amplified SSE light has been, in reality, measured. Namely, an error in measurement of the ASE light level P ASE caused by the SSE light has been great and it has been difficult to accurately measure the ASE light level P ASE.
In addition, as a fitting function for assuming the ASE light level, the high-degree function such as a normal distribution curve, a Lorenz curve or the like is used. As shown in FIG. 8B, the ASE light level P ASE has been specified based on the ASE light spectrum P3(λ) data obtained by interpolating an appointed fitting function into the wavelength range of the wavelength mask range of ±ΔλMA around the center of the signal wavelength λS.
However, according to this method, an obtainable fitting function does not suit the original spectrum curve P2(λ). As a result, an approximation error in the optical spectrum due to the fitting function becomes large. It becomes difficult to accurately specify the ASE light spectrum P3(λ). Accordingly, it has been a difficult problem to accurately specify the ASE light level P ASE based on the specified ASE light spectrum P3(λ).
Furthermore, when signal light supplied to the optical amplifier is wavelength division multiplex (WDM) light in that a plurality of wavelengths (channels) is multiplexed, a noise figure NF for each channel is individually measured. Therefore certain levels of time and labor have been necessary for measurement.